added vsprintf() - sprintf with variable arguments list

[fronders]

Hi, thanks for the lib!

I know that size-unsafe functions are not to be used. In one of my projects some 3rd party lib used both vsprintf() and vsnprintf(). Which meant that all vsnprintf() were successfully replaced by your implementation, but vsprintf() was used from default newlib (in my case for STM32 MCU). This led to lot of confusion until I discovered there's no vsprintf() implementation.

That's why i added vsprintf() function just so that all of the [v][s][n]printf() options are supported and replaced by the lib.

P. S. I like to use this header file from GitHub to check for banned/unsafe functions :)

[eyalroz]

Hello @fronders ,

I just merged your PR into my fork: eyalroz#3

... and I also added some unit tests so that coverity doesn't frown at you like here. The fork also has some other people's contribution pushed in.